A conventional ultrasonic probe comprises a transducer pallet (hereinafter "transducer") which must be supported within the probe housing. A conventional transducer comprises a linear array of narrow transducer elements, each made of piezoelectric material. Typically, each transducer element has metallic coatings on opposing front and back faces to serve as ground and signal electrodes respectively. The signal electrodes are typically connected to respective electrical conductors formed on a flexible printed circuit board.
When a voltage waveform is developed across an electrode during transducer operation, the piezo-electric material vibrates at a frequency corresponding to that of the applied voltage, thereby emitting an ultrasonic wave into the media to which the piezoelectric element is coupled. Conversely, when an ultrasonic wave impinges on an element, the piezoelectric material produces a corresponding voltage across its terminals and the associated electrical load component of the electrical source.
The transducer also comprises a mass of suitable acoustical damping material having high acoustic losses positioned at the back surface of the element array. The backing layer is acoustically coupled to the rear surface of the transducer elements, via the acoustically transparent PCB, to absorb ultrasonic waves that emerge from the back side of each element so that those waves will not be partially reflected and interfere with the ultrasonic waves propagating in the forward direction.
Typically, the front surface of each transducer element of array is covered with at least one acoustic impedance matching layer. The impedance matching layer transforms the high acoustic impedance of the transducer elements to the low acoustic impedance of the human body and water, thereby improving the coupling with the medium in which the emitted ultrasonic waves will propagate.
The transducer element array, backing layer and acoustic impedance matching layer are all bonded together in a stack-up arrangement. A lens material is then cast around the transducer stack-up. During assembly of the ultrasonic probe, the transducer stack-up with lens cast thereon is secured in the probe housing using adhesive.
In conventional applications, each transducer element produces a burst of ultrasonic energy when energized by a pulsed waveform received from a transmitter in the associated imaging system via a coaxial wires connected to the flexible PCB. This ultrasonic energy is transmitted by the probe into the tissue of the object under study. The ultrasonic energy reflected back to transducer element array from the object under study is converted to an electrical signal by each receiving transducer element and sent separately to a system receiver, again via the coaxial wires and the PCB. The release of acoustic energy during transmission creates a thermal buildup in the probe due to acoustic losses being converted into heat. The amount of heat that can be allowed to build up on the exterior of an ultrasound probe must be within prescribed limits. Typically the limit is that the temperature on the patient contact surface of the probe cannot exceed 16.degree. C. above ambient temperature or 41.degree. C., whichever is smaller. Most of the heat tends to build up immediately around the transducer elements, which are necessarily situated in the probe very close to the body of the patient being examined.
Conventional thermal management in ultrasound probes is accomplished with relatively simple devices such as heat pipes, which are buried in the transducer structure so that they transfer heat from the source into the body of the probe structure as quickly as possible. In this way heat is piped from the critical front surface of the probe into the handle where the increased mass helps dissipate the heat evenly.
For example, U.S. Pat. No. 5,545,942 discloses the employment of foil heat conductors made of heat conductive, electrically nonconductive material. The foil heat conductors are placed around the periphery of the transducer (but within the probe housing) so that heat can be drawn away from the transducer face and toward the rear/interior of the probe. These heat conductors act as conduits for draining heat from the thermal potting material which fills the spaces inside the probe housing. Thus, the heat conductors are effectively thermally coupled to the transducer element array. This arrangement increases the ability to dissipate heat away from the transducer and thus away from the patient being examined. U.S. patent application Ser. No. 08/343,063 also discloses that the internal heat pipes can be thermally coupled to the shielding braid of the cable. Because the shielding braid is made of tin/copper, connecting the heat pipes to the shielding braid facilitates the wicking away of even more heat from the transducer element array. By soldering the overall shield into a metal foil structure which is in contact with the internal heat pipes, heat generated by the transducer can be piped into the cable and dissipated throughout the 2-m length of the cable. Internal potting with thermally conductive epoxy also helps provide additional contact to the shield and the individual shields of the individual signal coaxial wires inside the cable.
U.S. Pat. No. 5,721,463 discloses a device for improving thermal transfer inside an ultrasound probe and reducing heat buildup near the transducer face. The cable components are used as heat pipes which conduct heat out of the probe handle. The cable assembly in an ultrasonic probe is composed of multiple coaxial wires bundled together and covered with an overall braided shield. The shield is in turn encased by a cable jacket made of polymeric material. Each individual coaxial wire comprises a plurality of individual conductors surrounded by a twisted shield. These heat conductive structures serve as thermal transfer devices when thermally coupled to an internal heat pipe, made of a sheet or plate of heat conductive material, which is embedded in the backing layer material of the transducer. Thus, heat generated by the transducer array can be transferred, via the internal heat pipe and the cable heat pipes, away from the probe surface which contacts the patient. The coaxial cable also has a bundle of strands or fibers made of a high-tensile-strength polymeric material (e.g., nylon) arranged in the center of the cable (e.g., surrounded by a circular array of signal coaxial wire bundles) and running the length of the cable. This central bundle of nylon strands reinforces the cable to withstand tensile loads thereon.
Many conventional probes are also difficult to assemble and bulky, making them expensive to manufacture and uncomfortable to the patient and user during scanning. Previous probes used permanent adhesive that required special fixture and time for proper curing, and was messy to work with. Moreover, although probes which are not bulky have been made for specific applications, these probes are not repairable because of the permanent adhesive. Thus there is a need for a repairable compact ultrasound probe which is easy to assemble and disassemble. In addition, such a probe should have cable load transfer and heat dissipation capabilities.